There are several reasons for taking the time to make sure we choose the right gas when diving on trimix. It allows us to ensure we are diving on a safe PO2 and to control our levels of narcosis.

When we do choose our OC gasses there is guidance on what might or might not be a safe or appropriate gas, so let’s just review some of the generally accepted OC practices for choosing trimix. At the moment we will just look at our deep gas. Intermediate and decompression gasses will be looked at in a separate post.

If we look at the ‘pressure T’ we can determine the best mix, MOD or TOD or safe depth for a particular gas.PO2 is the partial pressure of oxygen, FO2 is the fraction of oxygen in the gas mix and P is the depth expressed an an absolute pressure. Whatever we wish to determine, we simply ‘cover that up’ and then do the remaining sum.

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PO2 = FO2 x P (to determine the PO2 of a gas at a depth)FO2 = PO2/P (to determine the best mix for a gas at a a particular partial pressure and depth)P = PO2/FO2 (to determine the MOD or TOD of a gas at a particular partial pressure)

For example, the absolute pressure ‘P’ at 30 metres is 4 bar, Nitrox 32 or 32% has an FO2 of .32 and 32% at 30 metres will have a PO2 of 1.28 (.32 x 4 Bar).

When choosing a Nitrox for shallower diving the staring point is usually the PO2 of the gas at our Target Operating Depth (TOD) or Maximum Operating Depth (MOD), depending on the nature of the dive.When choosing a Trimix we would have the same start point but with some adjustments. So let’s quickly recap the ‘starting PO2s’ we might opt for.

A PO2 of 1.6 is, as we know the maximum limit. Normally used in technical diving rather than recreational and typically only for decompression gasses.

A PO2 of 1.5 is often called the recreational limit.

A PO2 of 1.4 is similarly called the working limit and very commonly used as the start point for calculating the FO2 for the best mix for a trimix dive. There are some other adjustments which you might choose to adopt, again depending on the nature and exposure even of the dive.

In it’s simplest form with no adjustments or safety factors applies an example for a 60m dive would look like this.

60m = 7 Bar absolute

PO2 of 1.4/7 = .20 (or 20% O2)

So we now have an FO2 for your trimix.

We all know that there are a number of factors which have the potential to make us more susceptible to CNS oxygen toxicity. I’m not going to review them all but included amongst them is getting cold, increased CO2 production and the length of exposure to the elevated PO2. Hard work underwater will increase our CO2 production. So for cold, a hard working dive or a long dive we can reduce our start point PO2 for calculating our FO2. The reduction factor is 0.25 for each.

From the example above we might be planning a relatively straight forward 60m dive as far as bottom time goes but water is a bit chilly cold and there might be a current flowing so we could choose to reduce ur starting PO2 by 0.25 for each of these factors. This would give us a starting PO2 for our calculation of 1.35.

1.35/7 = .193 rounded up so we might choose .18 or .19 (18% or 19% O2) as our FO2 in our trimix.

As an aside, it is fairly standard within the dive industry that when a dive centre mixes nitrox or trimix then a variation of + or - 1% is acceptable. It is better to err on the side of caution and ask for a slightly lower FO2 so that if it comes out a tiny bit rich it is still going to be a safe gas.

We can, if wanted, apply a ‘safety factor to our decompression gasses too, of -0.05 Bar PO2 for each of cold or long dive time (there shouldn’t really be any hard work on deco!) This is an additional safety margin we can put in to control our CNS% and OTU loading on a longer dive.

Having chosen our FO2 we then need to calculate how much Helium we want in order to control our levels of narcosis. We perhaps should set aside for another debate the question of O2 being narcotic. Yes, the mathematics and physical properties of the gas suggest it should be but there are equally compelling agreements, based on scientific investigations, that suggest in real diving, practical terms, it is unlikely to actually cause narcosis. If I can dig out some research I can post a synopsis of some of the theories. Based on these documents I personally fall on the side of not taking in to account the potential for O2 to be narcotic so I will leave that out of the process of calculating the He to add in our mix.

The start point should be your own comfort levels as far as narcosis is concerned. Some are happy to dive to a deeper level of narcosis than others so it really can be a very person choice but we should try to keep some level of team compatibility if we can.

So let us choose 30 metres as our equivalent air depth (EAD). We need to calculate what the partial pressure of nitrogen (PN2) is at a depth of 30m. So, the absolute pressure at 30m is 4 Bar, so the PN2 is 4 x .79 = 3.16.

3.16 is the peak PN2 we are aiming for at our actual dive depth of 60m. If we divide 3.16 by the absolute pressure at 60m of 7 Bar (3.16/7 = 0.451) we get an FN2 of 0.45. This is how much N2 we will allow in the mix.

To calculate the FHe we simply subtract the FO2 and FN2 from the whole and what is left is the FHe.

So, 1 - .18 - .45 = .37. Our FHe in our mix is .37 (37%). Our gas mix is now calculate as Trimix 18/37. (It is common to give the FO2 first then the FHe when deciding a trimix gas).

So, all fairly straight forward. Bear in mind that this process is used for OC gas calculations. We would adopt a slightly different approach and apply slightly different rules when calculate a trimix for use in a CCR both for onboard active diluent and bailouts.